This application is the National Stage Application of PCT/JP00/00019 filed Jan. 6, 2000.
The present invention relates to corrosion inhibitor compositions for magnesium or magnesium alloys and a process for inhibiting the corrosion of such metals with use of the composition.
The present invention relates also to surface treating agents and a surface treating process for shaped articles of magnesium and/or magnesium alloy, and a process for producing components made from magnesium and/or magnesium alloy.
Magnesium is the most lightweight of all the metals for use as practically useful structural materials, has a high specific strength, is easy to machine and therefore has found wide use for motor vehicle components, electric products such as computers and acoustic devices, aircraft components, etc. Generally, magnesium and magnesium alloys are made into shaped articles mainly by die casting, extrusion or rolling, while the so-called thixomolding process with use of an injection molding machine has been established technically in recent years. This process assures the freedom of shape of moldings, the productivity thereof and improved properties, rendering the moldings useful for wider application.
However, magnesium is the basest of all the metals for use as practically useful structural materials, therefore has the drawback of being susceptible to oxidation and needs to be inhibited from corroding as an important problem.
Magnesium or magnesium alloys are generally treated with chromates for corrosion inhibition (for example, JP-B No. 17911/1986, etc.). The chromate treatment nevertheless involves difficulty in setting the conditions for the treatment, so that it has been desired to provide more convenient corrosion inhibiting processes. Furthermore, the chromate treatment has the drawback that when conducted, the treatment discolors the surface of the metal, depriving the metal of its luster. Since the treatment uses a chromium compound, processes are more desirable which are less likely to burden the environment.
Although magnesium and/or magnesium alloys are not very costly as materials, the shaped products of magnesium and/or magnesium alloys prepared by thixomolding, extrusion, rollingor die casting have a highly active surface, which therefore becomes corroded at a high rate, necessitating a cumbersome surface treatment. The cost of this treatment inevitably makes the product two to three times as expensive as resin molding conventionally in use.
Castings or molding obtained by die casting or thixomolding are made into magnesium alloy products generally by the following steps.
1. Mechanical Pretreating Step
Polishing step with use of a polishing belt, abrasive paper or brush or by barrel finishing, buffing, blasting or the like for removing surface roughness or extraneous matter such as burrs, tough oxides, extrusion lubricant, mold releasing agent, casting sand, cutting oil or common soil.
2. Degreasing Step
(1) Degreasing with solvent: Preliminary degreasing or cleaning for removing cutting oil, grease or the like with a petroleum, aromatic, hydrocarbon or chlorine solvent.
(2) Degreasing with alkali: Degreasing or cleaning with use of caustic soda or like alkali solution for removing common soil, scorched lubricant or cutting oil, etc.
(3) Degreasing with emulsion: Cleaning for removing soil from the metal surface by emulsification.
3. Pickling Step
The step of cleaning with a solution of single acid such as hydrofluoric acid, nitric acid, phosphoric acid or chromic acid or a solution of a mixture of such acids for removing oxide film, corrosion product, scorched lubricant, lodged abrasive agent, shot, casting sand or other soil which remains unremoved by the degreasing step, activating the surface of the casting or molding, or removing segregated layer.
4. Step of Chemical Conversion Treatment
The step of forming a chromate film over the surface of the casting or molding generally with use of a chromic acid agent to give corrosion resistance.
5. Drying Step
6. Coating or Plating Step
7. Assembling Step
Since magnesium is the basest of all the practically useful structural materials and has properties susceptible to oxidation, the magnesium casting or molding obtained by die casting or thixomolding requires many steps when to be made into a product for use as a component of magnesium alloy, necessitating equipment, chemical agents, labor, etc. for the steps and consequently leading to reduced productivity and an increased cost.
These steps each have drawbacks as will be described below.
1. The mechanical pretreating step produces cut chips or fine particles of magnesium due to polishing, involving the hazard of ignition or explosion and necessitating utmost care for the work.
2. The degreasing step requires good care for the disposal of waste liquid or waste water in view of the influence on the environment. Especially the release of solvents, such as chlorine solvents, which are likely to be toxic to the environment must be avoided, hence the need for a limitation on use.
3. The pickling step produces marked dimensional variations in the casting or molding.
4. The step of chemical conversion treatment, especially of chromate treatment, (1) is likely to exert an influence on the environment, (2) discolors the treated surface, depriving the surface of the metallic luster, and (3) reduces the purity of magnesium owing to contamination with chromium when the product is recycled.
The coating step has a problem as to the adhesion between the magnesium or magnesium alloy substrate and the coating formed thereon. Although the chromate film gives improved adhesion to the coating, chemical conversion treating agents of the nonchromate type are desired because of the reasons given above and the worldwide trend to impose a limitation on the use of hexavalent chromium. Presently manganese phosphate is proposed as a chemical conversion treating agent of the nonchromate type, whereas the presence of manganese in this agent is not desirable from the viewpoint that this impurity metal becomes incorporated into magnesium recycled, and manganese adversely affects the electromagnetic wave shielding properties of magnesium or magnesium alloy which are characteristic thereof although the proposed compound is almost satisfactory with respect to the adhesion of the coating.
An object of the present invention is to provide a corrosion inhibitor composition which is convenient for use in the anticorrosion treatment of magnesium or magnesium alloy while permitting the metal to retain its metallic luster despite the treatment, and which is less likely to involve environmental problems, and to also provide a process for inhibiting corrosion with use of the corrosion inhibitor composition.
Another object of the invention is to provide a surface treating agent and a surface treating process for shaped products of magnesium and/or magnesium alloy which can be used or practiced with a reduced number of steps and smaller equipment, decreased amounts of chemical agents and diminished labor to achieve improved productivity and a greater cost reduction, and also a process for producing magnesium and/or magnesium alloy components.
Still another object of the invention is to provide a surface treating agent which gives improved adhesion to coatings and produces high corrosion inhibitory effects without resulting in impaired properties to shield electromagnetic waves.
The present invention provides a corrosion inhibitor composition for magnesium or magnesium alloys which contains at least one compound selected from among aromatic carboxylic acids and salts thereof as an effective component.
The invention further provides a corrosion inhibitor composition for magnesium or magnesium alloys which contains at least one compound selected from among aromatic carboxylic acids and salts thereof, and at least one compound selected from among pyrazole compounds and triazole compounds.
The invention further provides a process for inhibiting corrosion of shaped magnesium articles characterized in that a molding or casting prepared from magnesium or a magnesium alloy by thixomolding or die casting is coated over the surface thereof with one of the above corrosion inhibitor compositions.
The invention further provides a surface treating agent for magnesium and/or magnesium alloy components which contains a phosphate and, at least one compound selected from among aromatic carboxylic acids and salts of the acids.
For use in surface-treating magnesium and/or magnesium alloy components, the invention provides a process for surface-treating magnesium and/or magnesium alloy components which is characterized by using a surface treating agent containing a phosphate and, at least one compound selected from among aromatic carboxylic acids and salts of the acids.
The invention further provides a process for treating magnesium and/or magnesium alloy components which process is characterized by treating the component with the surface treating agent and thereafter treating the component with the corrosion inhibitor composition.
The invention further provides a process for producing magnesium and/or magnesium alloy components with use of the surface treating agent and the surface treating process.
Given below are preferred embodiments of the invention.
(1) A corrosion inhibitor composition wherein the aromatic carboxylic acid and the salt thereof are cuminic acid, o-cuminic acid, m-cuminic acid, p-tert-butylbenzoic acid, m-toluic acid, o-toluic acid or p-toluic acid, and an alkanolamine salt of such an acid.
(2) A corrosion inhibitor composition wherein the triazole compound is 1,2,3-triazole or 1,2,4-triazole.
(3) A surface treating agent wherein the phosphate is at least one of ammonium salts or alkanolamine salts of phosphoric acids.
(4) A surface treating agent wherein the phosphate is an ammonium salt of condensed phosphoric acid (ammonium condensed phosphate).
(5) A surface treating process wherein the phosphate is at least one of ammonium salts or alkanolamine salts of phosphoric acids.
(6) A surface treating process wherein the phosphate is an ammonium salt of condensed phosphoric acid.
The corrosion inhibitor composition of the present invention contains at least one compound selected from among aromatic carboxylic acids and salts thereof. The aromatic carboxylic acid to be used is preferably a compound of the formula (1) which is substituted with R1 at the first position of its benzene ring and with R2, R3 or R4 at any one of the 2- to 6-positions of the ring, or a compound of the formula (2) which is substituted with R1 at the first position of its naphthalene ring, with R8 at the 8-position of the ring and with R2, R3, R4, R5, R6 or R7 at any one of the 2- to 7-positions. 
wherein R1 is carboxyl, carboxymethyl or carboxyvinyl, R2, R3, R4, R5, R6 and R7 are the same or different and are each a hydrogen atom, hydroxyl, C1-C8 alkyl, nitro, a halogen atom or amino, and R8 is a hydrogen atom, hydroxyl, carboxyl, carboxymethyl or carboxyvinyl.
Such aromatic carboxylic acids and salts thereof are compounds having a high corrosion inhibitory effect on magnesium and/or magnesium alloys, causing no surface discoloration and producing no influence on the subsequent treating step.
Specific examples of such carboxylic acids are benzoic acid, cuminic acid, o-cuminic acid, m-cuminic acid, p-tert-butylbenzoic acid, m-toluic acid, o-toluic acid, p-toluic acid, hydroxytoluic acid, mononitrobenzoic acid, dinitrobenzoic acid, nitrotoluic acid, nitrophthalic acid, chlorobenzoic acid, p-nitrophenylacetic acid, nitrocinnamic acid, naphthoic acid, 2-hydroxynaphthoic acid, naphthalic acid, etc.
Usable as salts of these acids are salts of such acids with various organic bases and inorganic bases. Examples of organic bases are monoethanolamine, diethanolamine, triethanolamine, monoisopropanolamine, diisopropanolamine, triisopropanolamine and like alkanolamines, methylamine, ethylamine and like alkylamines, and cyclohexylamine, DBU(1,8-diazabicyclo[5.4.0]-7-undecene), DBN(1,5-diazabicyclo[4.3.0]-5-nonene), 1-aminopyrrolidine, morpholine and like cyclic amines. Examples of inorganic bases are ammonia, TMAH (tetramethylammonium hydroxide)-and like ammonias, hydrazine, sodium hydroxide, potassium hydroxide and like alkali metal hydroxides. One of such salts is usable singly, or at least two of them are usable at the same time. These salts are more soluble in water, have a higher corrosion inhibitory effect and are therefore more preferable than aromatic carboxylic acids used as such without conversion to salts.
Among these salts, alkanolamine and like organic amine salts, ammonia salts and hydrazine salts are especially preferred because crystals will not adhere to the surface of the article treated with use of such a salt and further because these salts give satisfactory surface properties.
Examples of especially preferable aromatic carboxylic acids and salts thereof for use in the present invention are cuminic acid, o-cuminic acid, m-cuminic acid, p-tert-butylbenzoic acid, m-toluic acid, o-toluic acid, p-toluic acid, and alkanolamine salts of these acids.
It is desirable to use a pyrazole compound or triazole compound in combination with the aromatic carboxylic acid from the viewpoint of giving an improved corrosion inhibiting property to the corrosion inhibitor composition of the invention.
Examples of such pyrazole compounds are pyrazole and pyrazole derivatives having a pyrazole ring substituted with hydroxyl, C1-C8 alkyl, amino or nitro at the 3- to 5-positions of the ring.
More specific examples of useful pyrazole compounds are pyrazole, 3,5-dimethylpyrazole, 3-methyl-5-hydroxypyrazole, 4-aminopyrazole, etc.
Examples of such triazole compounds are 1,2,3-triazole, 1,2,4-triazole, benzotriazole and like triazole compounds, and triazole derivatives comprising such a triazole compound substituted with C1-C8 alkyl, mercapto, hydroxyl or the like at a desired position.
More specific examples of such triazole compounds are 1,2,3-triazole, 1,2,4-triazole, 3-mercapto-1,2,4-triazole, 3-hydroxy-1,2,4-triazole, 3-methyl-1,2,4-triazole, 1-methyl-1,2,4-triazole, 1-methyl-3-mercapto-1,2,4-triazole, 4-methyl-1,2,3-triazole, benzotriazole, 1-hydroxybenzotriazole, etc. Especially preferable among these are 1,2,3-triazole, 1,2,4-triazole, 3-mercapto-1,2,4-triazole, 3-hydroxy-1,2,4-triazole and benzotriazole, and more preferable are 1,2,3-triazole and 1,2,4-triazole. These pyrazole compounds or triazole compounds are usable singly, or at least two of them can be used at the same time.
The composition of the present invention is usable as it is or as dissolved in a suitable solvent, while it is desirable to use the composition in the form of an aqueous solution.
Although aromatic carboxylic acids and salts thereof can be incorporated into the composition of the invention in amounts determined suitably, the combined amount of such compounds can be, for example, usually 0.01 to 30 wt. %, preferably 0.1 to 10 wt. %.
When to be used, the pyrazole compound or triazole compound is used in an amount of 0.01 to 30 wt. %, preferably 0.1 to 10 wt. %. The ratio by weight of the aromatic carboxylic acid and salt thereof to the pyrazole compound or triazole compound can be, for example, 10:1 to 1:10.
The magnesium or magnesium alloy for which the corrosion inhibitor composition of the present invention is usable is not limited specifically. The composition is usable for magnesium as a single metal and a wide variety of alloys or composite materials comprising magnesium and other metals. Examples of other metals are aluminum, zinc, manganese, iron, nickel, copper, lead, tin and calcium. One or at least two metals can be selected from among these metals for use.
The corrosion inhibitor composition of the invention can be applied to the surfaces of ingots, chips or various shaped articles to be treated, by spraying, coating with a roll coater or impregnation with use of a treating bath. The temperature for the corrosion inhibiting treatment, which is suitably determined, is usually 0 to 100xc2x0 C., preferably room temperature to about 80xc2x0 C.
When the molding or casting obtained by thixomolding or die casting is treated over the surface thereof with the corrosion inhibitor composition of the invention, the molding or casting can be distributed or stored for a long period of time before coating. This contributes greatly to the rationalization of the manufacturing process. The magnesium alloy molding or casting conventionally prepared by thixomolding or die casting (hot-chamber die casting and cold-chamber die casting) has its surface corroded at a high rate and therefore needs to be coated immediately after preparation, or to be treated temporarily with a corrosion inhibitor which is to be removed before coating, whereas the article surface treated with the corrosion inhibitor composition of the invention can be directly coated free of any adverse influence of the composition, so that there is no need for the removal step conventionally required.
To achieve an enhanced inhibitory effect, the article to be treated is preferably degreased and cleaned over the surface before the inhibitor composition of the invention is used.
The amount of the corrosion inhibitor composition of the present invention to be used is not limited specifically but may be such that the surface of the article to be treated can be uniformly covered with the composition. For example, the composition can be used in an amount of about 10 to about 300 ml per square meter of the surface to be treated.
The ingot or chips as treated with the corrosion inhibitor composition of the invention can be used as it is as the material to be shaped without removing the composition. The shapability of the material or the shaped product is then in no way adversely affected by the composition.
When the corrosion inhibitor composition of the invention is used for shaped articles, the article having the composition applied thereto can be coated directly without providing the step of removing the composition, hence the outstanding advantage that the coated article can be very easily prevented from developing corrosion or becoming discolored.
Examples of phosphates for use in the surface treating agent of the present invention are alkali metal salts, ammonium salts and alkanolamine salts of orthophosphoric acid, condensed phosphoric acids or like phosphoric acids.
Examples of condensed phosphoric acids are metaphosphoric acids and polyphosphoric acids. Examples of metaphosphoric acids are trimetaphosphoric acid, tetrametaphosphoric acid, etc. Examples of polyphosphoric acids are pyrophosphoric acid, triphosphoric acid, tetraphosphoric acid and the like.
More specific examples of phosphates are sodium primary phosphate, sodium secondary phosphate, sodium tertiary phosphate, potassium primary phosphate, potassium secondary phosphate, potassium tertiary phosphate, ammonium primary phosphate, ammonium secondary phosphate, ammonium tertiary phosphate, monoethanolamine salt of phosphoric acid, diethanolamine salt of phosphoric acid, triethanolamine salt of phosphoric acid, isopropanolamine salt of phosphoric acid, sodium salt of trimetaphosphoric acid, potassium salt of trimetaphosphoric acid, ammonium salt of trimetaphosphoric acid, sodium salt of tetrametaphosphoric acid, ammonium salt of tetrametaphosphoric acid, ethanolamine salt of tetrametaphosphoric acid, sodium salt of triphosphoric acid, potassium salt of triphosphoric acid, ammonium salt of triphosphoric acid, sodium salt of tetraphosphoric acid, potassium salt of tetraphosphoric acid, ammonium salt of tetraphosphoric acid, etc. These phosphates can be used singly, or at least two of them are usable in combination.
Among these, ammonium salts and alkanolamine salts of phosphoric acids are desirable since they have a suitable etching effect and are less likely to produce smut after cleaning. More desirable are ammonium salts of condensed phosphoric acids because they have high safety, permit facilitated waste water disposal, are capable of readily etching the surface of magnesium and/or magnesium alloy and are unlikely to etch to excess.
The ammonium salts of condensed phosphoric acids are known. Such a salt can be obtained, for example, by heating orthophosphoric acid (normal phosphoric acid) and urea for condensation. In this case, the reaction is conducted preferably under such a condition that the molar ratio of orthophosphoric acid to urea is 1:0.5 to 1:5. The surface treating agent may contain the unreacted materials in the reaction mixture, i.e., orthophosphoric acid and urea, and is usable without giving any problem to the advantage of the invention. The degree of condensation of the ammonium salt of condensed phosphoric acid is not limited particularly, but the acid may have a condensation degree of about 2 to about 3.
The phosphoric acid salt is used usually in an amount of about 0.5 to about 50 wt. %, preferably about 2 to about 5 wt. %, based on the whole amount of the surface treating agent of the present invention. If the amount is much greater than 50 wt. %, the surface of magnesium becomes colored black after cleaning, whereas if the amount is less than 0.5 wt. %, insufficient etching will result, failing to produce a full degreasing effect.
Examples of aromatic carboxylic acids and salts thereof for use in the surface treating agent of the present invention can be aromatic carboxylic acids represented by the foregoing formula (1) or (2), and salts thereof.
Preferred aromatic carboxylic acids, more specific examples such acids, salts of such aromatic carboxylic acids, and preferred examples of such salts are the same as those given above.
The concentration of aromatic carboxylic acids and salts thereof for use is usually about 0.01 to about 30 wt. %, preferably about 0.1 to about 10 wt. %, based on the whole amount of the surface treating composition. If the concentration is much higher than 30 wt. %, the surface treating agent will exhibit a lower etching rate, necessitating a longer period of time for the treatment, whereas if the concentration is lower than 0.01 wt. %, the surface treating agent colors the surface of magnesium black and fails to produce a sufficient effect although etching the metal progressively. The composition can be produced, stored and transported with its components held at high concentrations, and is to be diluted for actual use.
At least one compound selected from among pyrazole compounds and triazole compounds can be used in combination with the aromatic carboxylic acid and salt thereof in the surface treating agent of the present invention. The same compounds as exemplified above are usable as pyrazole compounds and triazole compounds.
The ratio by weight of aromatic carboxylic acids and salts thereof to the pyrazole compound or triazole compound can be, for example, 10:1 to 1:10. It is desirable to use the pyrazole compound or triazole compound in combination with the aromatic carboxylic acid and salt thereof from the viewpoint of giving synergistically improved corrosion inhibitory properties.
Various additives, such as surfactants and chelate agents, can be incorporated into the corrosion inhibitor and the surface treating agent of the present invention. The surfactant is preferably nonionic and is about 13 to about 20 in HLB value to be suitable. The concentration of the surfactant, although determined suitably, is usually 0.001 to 5 wt. %, preferably about 0.01 to about 3 wt. %. Examples of useful chelate agents are disodium salts of ethylenediaminetetraacetic acid (EDTA-2Na), sodium gluconate, phosphonic acid salts, etc. The concentration of the chelate agent, although determined suitably, is usually 0.1 to 10 wt. %, preferably about 1 to about 5 wt. %.
Although the surface treating composition of the present invention can be used as it is or as dissolved in a suitable solvent, it is desirable to use the composition in the form of an aqueous solution. The temperature for the treatment, which is suitably determined, is usually 0 to 100xc2x0 C., preferably room temperature to about 60xc2x0 C.
The magnesium or magnesium alloy for which the surface treating composition of the present invention is usable is not limited specifically. The composition is usable for magnesium as a single metal and a wide variety of alloys or composite materials comprising magnesium and other metals. Examples of other metals are aluminum, zinc, manganese, iron, nickel, copper, lead, tin and calcium. One or at least two metals can be selected from among these metals for use.
The material to be treated is treated with the surface treating agent of the invention and then washed with water when required. Preferably, the material is thereafter cleaned with a solvent for removing from the surface fine particulate substances such as metal powder and carbon. Examples of solvents usable are methanol, ethanol, isdpropanol and like alcohols, acetone, methyl ethyl ketone and like ketones, trichloroethane, trichloroethylene, Perclene and like chlorine-containing solvents, limonene and like terpenes, aqueous solutions of alkalis such as sodium hydroxide, potassium hydroxide, sodium orthosilicate and sodium metasilicate, etc. These solvents can be used preferably at a concentration of 1 to 100%, preferably 5 to 50%, and at a temperature of room temperature to 100xc2x0 C., preferably room temperature to 50xc2x0 C.
The surface treating composition of the present invention can be applied to the surfaces of shaped articles prepared as by thixomolding, extrusion, rolling or die casting, for example, by spraying, coating with a spray or roll coater, or impregnation with use of a treating bath.
According to the present invention, the surfaces of shaped articles of magnesium and/or magnesium alloy prepared as by thixomolding, extrusion, rolling or die casting can be readily cleaned and treated for corrosion inhibition. This greatly simplifies the conventional manufacturing process for producing components of magnesium and/or magnesium alloy.
More specifically, some or all of the conventional degreasing step, pickling step and chemical conversion treatment step described can be replaced by the treatment with the surface treating composition of the present invention. Further in the mechanical pretreatment following casting, molding or shaping process, the treatment for giving a uniform surface, cleaning or corrosion inhibition can be replaced by the treatment with the surface treating agent of the invention although it may be necessary to conduct, for example, deburring treatment.
Briefly stated, parts or components of magnesium and/or magnesium alloys can be produced ideally according to the invention from moldings or castings of magnesium and/or magnesium alloys obtained by thixomolding or die casting, by (1) deburring the molded or cast articles when required, (2) treating the articles with the surface treating agent of the invention, (3) washing the articles with water and treating the articles for corrosion inhibition when required, (4) drying the articles, (5) coating or plating the articles or treating the articles for anodic oxidation, and (6) thereafter assembling the articles.
Incidentally, the articles washed with water by step (3) can be treated thereafter with a corrosion inhibitor. This further improves the corrosion inhibitory or like surface protecting effect to be producedby the following step (5) of coating, plating or the like on the magnesium and/or magnesium alloy articles. Examples of useful corrosion inhibitors are the aromatic carboxylic acids, salts thereof, pyrazole compounds and triazole compounds described above for use in the invention. It is desirable to use an aqueous solution containing at least one of these compounds. While the amount of the inhibitor to be used should be adjusted suitably depending on the kind thereof, the amount is generally 0.01 to 30 wt. % based on the whole amount of the corrosion inhibitory solution. The solution is applied to the magnesium and/or magnesium alloy articles as washed with water by spraying, coating with use of a roll coater or dipping in the solution.
It has become possible to solve or relieve the problems involved in the conventional surface treating step. Additionally, the invention improves the equipment conventionally required, decreases the amounts of chemicals, labor, etc. to be used in various step, and is expected to achieve improved productivity and cost reductions.
When treated with the surface treating composition of the present invention, moldings or castings can be distributed or stored before being coated or plated, while the film formed on the surface by the treatment will not be adversely affected by the coating, plating or anodic oxide film to be subsequently formed over the treated surface. This eliminates the need for a removal step, contributing to further rationalization of the process for producing magnesium and/or magnesium alloy parts or components. When magnesium substrates are directly coated conventionally, the adhesion of the coating poses a problem, whereas the film formed on the surface by the treatment gives satisfactory adhesion to the coating.